An internal combustion (IC) typically employs a valve train to convert rotary lift of a camshaft to linear lift of an engine poppet valve to enable a gas exchange process. Precise control of a valve lift event is required for consistent performance, emission control, and durability. To enable such precise control, clearances between valve train components must be maintained throughout the life of the engine. The summation of the clearances between valve train components is typically in the form of a resultant clearance or gap between the tip of the valve and the adjacent valve train component acting on the valve. This resultant clearance, often called “valve lash” must compensate for thermal growth of the valve and wear at each of its two interfaces over the life of the IC engine. Too high of a valve lash can result in unwanted wear, noise and undesirable performance of the engine, while too low of a valve lash can cause the valve to be inadvertently opened when it should be closed.
Valve lash can be mechanically adjusted, for example, by a threaded valve interface and jam nut combination arranged within the valve train component that actuates the valve. However, periodic valve lash adjustments throughout the life of the IC engine must be made to accommodate engine wear.
Many of today's valve trains employ a hydraulically controlled lash compensator that automatically adjusts to the dimensional and thermal variations of the valve train components to provide a zero valve lash condition throughout the life of the IC engine, eliminating the need for periodic valve lash adjustments. A component of the lash compensator is an axially displaceable piston configured with a control valve assembly that manages the exchange of hydraulic fluid between a high pressure chamber and a low pressure reservoir. Different configurations of the control valve assembly are possible. One such configuration is a reverse-spring design shown in FIGS. 12 and 13, contained within a hydraulic pivot element 110. Reverse-spring designs typically employ a bias spring 134 that engages a top portion of a closing body 142 of a control valve assembly 130, providing for a biased-open configuration. Conversely, traditional control valve configurations are configured such that the bias spring 134 engages a bottom portion of the closing body 142, providing for a biased-closed configuration. Reverse-spring designs offer advantages over traditional designs in some instances where inadvertent actuation of the engine poppet valve occurs. Such inadvertent actuation can be caused by camshafts with high base circle runout, dynamic tilt of a camshaft, or a pump-up condition of the lash compensator. Reverse-spring control valve designs depend on tightly controlled design tolerances and clearances to provide for repeatable valve lift events.
Referring to the reverse spring design of FIGS. 12 and 13, engagement of the closing body 142 with a valve seat 144 formed on a bottom surface 131 of the piston 126 occurs when a resultant fluid force F2 acting on the closing body 142 overcomes a bias spring force F3. As evident in FIG. 13, hydraulic fluid flow between the closing body 142 and closing body seat 144 through a flow crevice FC occurs before closure. This flow crevice FC, including the inherent restriction caused by the presence of the bias spring 134, affects the magnitude of the resultant fluid force F2 available to overcome the force F3 of the bias spring 134. The bias spring 134 is typically in the form of a compression spring configured with coils, as shown in FIGS. 12 and 13. Any variation of coil windings, particularly at an end of the bias spring 134 that makes contact with the closing body 142, influences the flow of hydraulic fluid 128 through the flow crevice FC and, thus, the generated fluid force F2. As variation in compression spring end-coil geometry is quite typical with current manufacturing methods, variation of fluid flow forces on the closing body 142 (caused by flow impingement on the spring end-coils) near the flow crevice FC can exist within an engine population of hydraulic pivot elements; this variation in flow induced forces on the closing body 142 near the flow crevice FC can yield a variation in the closing body 142 response time and valve lift amongst the engine valves of an internal combustion engine. As such a variation can negatively impact engine performance and exhaust emissions, a solution is needed to minimize or eliminate bias spring geometry effects on reverse-spring hydraulic lash adjuster performance.